Ultrasonic imaging advantageously enables real-time visualizing of structures within a human or animal body such, for example, as biological tissue, as well as the measurement and imaging of blood and other fluid flow velocities in internal body structures such as blood vessels and heart cavities. As heretofore practiced, a transducer is typically placed noninvasively on the skin or within a naturally-accessible internal cavity such as the esophagus, the vagina and the rectum. The ultrasonic beam is then scanned across the external or internal wall or skin or tissue surface to perform pulse echo reflection imaging of tissue structures in the skin or cavity wall and of structures adjacent to the cavity--e.g. the heart, spleen, prostate and uterus.
For intracavital applications, various kinds of arrays--such as switched linear or curvilinear array, and linear phase array types--have been employed. The use of annular arrays is particularly advantageous in such applications since they provide a circularly symmetrical dynamic focus and it is fairly simple to do continuous wave (CW) Doppler measurements of blood velocities using a steerable, albeit mechanically steerable, ultrasonic beam. In addition, because their individual elements are wider it is easier to fabricate such transducers for use at higher ultrasonic frequencies than with linear phased arrays. And annular arrays exhibit increased sensitivity for Doppler measurements and for imaging of blood velocities.
Phased and linear arrays, on the other hand, present problems in operating at relatively high ultrasonic frequencies--i.e. in the range of 7 to 10 MHz. They also provide electronically steered focusing only in the scan plane, the focus being fixed normal to that plane, whereas annular arrays provide improved lateral resolution normal to the scan plane.
Annular arrays, however, are disadvantageous in applications such as those contemplated for the present invention because they require that beam scanning be performed by mechanically or otherwise physically moving the transducer, as through a predetermined wobbling or rotative motion. This has proven particularly problematical in applications appropriate for or necessitating intracavital insertion of the probe since, for the highly miniaturized constructions necessary to accommodate their insertion into the body, prior art ultrasonic probes have been unable to attain sufficiently accurate control of the mechanical movement of the beam for these applications--especially where a wobbling motion of the beam is desireable or required. This deficiency has seriously hampered the fully effective use of ultrasonic probes during surgery--as, by way of example, for in situ observation of undissected structures such as tumors and atheroma in vessels, and for the measurement and imaging of blood velocities within vessels and cardiac cavities to provide both pre-procedure guidance for and ongoing control of surgical operations and evaluations.